You were very wise to stop when you did. Hypothermia is nothing to fool around with. You did a lot. Enjoy what you accomplished.
But... this is why I won't sign up for a timed event. If it's not a race, I want to be able to go as fast as I can, *for me.* If I am raising money for a good cause (rare, I hate to ask for $), I'm going to do it under comfortable conditions. I've ridden in the kind of rain you describe, on a tour, with big hills. While my rain gear worked, I knew I could get in the van at any time, and no one was timing me.